Time
Julian Date Converter
Convert a calendar date and UTC time to a Julian Date, or a Julian Date back to the calendar, with MJD, day of week, and day of year.
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Years -4712 through 9999, astronomical numbering: year 0 is 1 BC, year -1 is 2 BC, year -4712 is 4713 BC. Dates before 15 October 1582 are read in the Julian calendar.
24 hour UTC time. Seconds may include decimals down to 0.001 s.
A decimal number of days, for example 2451545.0. Supported range: 0 (noon UTC, 1 January 4713 BC) through 5373484.49999 (end of year 9999).
Result
- Julian Date (UTC)
- Modified Julian Date
- Truncated Julian Date
- Julian Day Number
- Calendar date
- Time (UTC)
- Also written
- Day of week
- Day of year
How this converter works
The Julian Date is a continuous count of days and fractions of a day since noon Universal Time on 1 January 4713 BC in the Julian calendar (JD 0.0). Astronomers use it so that intervals between observations become simple subtractions, unbroken by months, years, or calendar reforms. Both conversion directions here use the standard algorithms from Jean Meeus, Astronomical Algorithms, 2nd edition, chapter 7, evaluated exactly with integer date arithmetic, so the date part carries no series or truncation error at all. The fraction of a day is computed from the UTC time you enter and is shown rounded to 5 decimal places, a resolution of 0.864 seconds; internally the time is kept to the millisecond, so a Julian Date converted to a calendar date and back reproduces the original value exactly at the displayed precision.
Following astronomical convention, dates on or before 4 October 1582 are interpreted in the Julian calendar and dates on or after 15 October 1582 in the Gregorian calendar. The ten dates in between, 5 through 14 October 1582, never existed and are rejected with a message rather than silently adjusted. Years use astronomical numbering, in which year 0 is 1 BC and year -1 is 2 BC, so year -4712 is 4713 BC; results show both notations. The day-of-year count reflects real elapsed days, so 31 December 1582 is day 355 of that shortened year. Supported instants run from JD 0 (noon UTC, 1 January 4713 BC) through the end of year 9999.
The Modified Julian Date is MJD = JD - 2400000.5, counting from midnight UTC on 17 November 1858, and the Truncated Julian Date shown here is TJD = JD - 2440000.5, counting from midnight UTC on 24 May 1968, the NASA convention listed in the Explanatory Supplement to the Astronomical Almanac (a rarer four-digit recycling convention also exists, and is not used here). The Julian Day Number is the whole number assigned to the civil day, anchored at its noon. The day of the week comes from the Julian Day Number modulo 7, which is why JD 0 falling on a Monday fixes every other weekday in history.
All values describe the UTC instant you enter and every day is treated as exactly 86400 seconds, the normal Julian Date convention; leap seconds are not modelled, and dynamical time scales such as TT differ from UTC (by roughly 69 seconds in 2026), so an ephemeris JDE is not the same as the JD of a UTC clock reading. Everything is computed in your browser and nothing you enter leaves your device. Values are for planning, records, and education, not navigation or other safety-critical use.